Which GPU toPurchase? Nvidia Quadro k2200 or gtx 980?
Never did any actual real world testing, but from my limited experience and from what I've heard NVidia Quadros (or any "professional" grade cards) are never worth the extra money.

Besides that in Blender's specific case they are probably not worth it because they are likely not optimized, simply because most blender developers don't have testing access to Quadro card so even if they work at all, there's a good chance that there's little to no performance benefit at all. Most likely you would not get your price bump's worth in performance gain at least.

I have no benchmarks to back my theory up though,  so I advise you to do some actual investigation in this area by searching for "Cycles Cuda benchmarks"  or something along those lines. 

I own a 980ti and it works very well in parallel with my older gtx580.

I'd say you are probably better off buying a regular consumer card like the 980ti or even a TitanX if you need the extra memory for complex scenes.

If you need extra performance buy multiple cards and assemble them in a non SLI configuration. Blender Cycles scales really well with parallel cards. 

You may also look into the newer faster and cheaper GTX1080 which is expected to preform very well. But have in mind that because it is still a very new hardware it is not yet fully supported in Blender, this will likely change in future versions, though when that might happen is not known yet.